1. Related Applications
The subject matter of the present application is related to the subject matter of the applications entitled "Automatic Colorant Mixing Method and Apparatus" (attorney docket no. 100218) and "Device Independent Color Controller and Method" (attorney docket no. 100214) filed by the same assignee, which are incorporated herein by reference.
2. Field of Invention
The present invention is directed to controlling the colors of an image output by a marking device capable of outputting one or more colors, and in particular, to a method and apparatus for controlling an output color by correcting for color drift, i.e., the variation between the current output color and the same color output under standard conditions.
3. Description of Related Art
Users of color or black and white marking devices, e.g., printers, copiers, etc., increasingly demand highly accurate output. Users have also become accustomed to printers and copiers that produce high quality color and gray-scaled output. Users now expect to be able to reproduce a color image with consistent quality on any compatible marking device, including another device within an organization, a device at home or a device used anywhere else in the world.
Over time, the output of conventional marking devices drifts (or deviates from predetermined optimum standards) due to various factors. These factors include environmental conditions (temperature, relative humidity, etc.), use patterns, the type of media (e.g., different paper types, transparencies, etc.) used, variations in media, variations from original models used in initialization, general wear, etc. When a marking device is originally initialized, and at regular or irregular intervals thereafter, it is calibrated to produce output as close as possible to a reference standard. The calibration process, however, is time consuming and expensive, particularly because specific expertise is required.
As an example, calibration of a conventional four-color (cyan, magenta, yellow and black) printer or copier involves at least the following processes in sequence (see FIG. 3): (1) generating a 3D look-up table for mapping device independent parameter space to CMY (cyan-magenta-yellow) space (step S302); (2) executing a GCR (gray component replacement)/UCR (under color removal) strategy (and possibly gray balancing) to convert the CMY space parameters to CMYK space parameters which represent the colors of a typical four-color marking device (step S304); (3) linearizing marking device TRCs (tone reproduction curves) to account for marking device variabilities (normally done at the time of manufacturing or whenever the printer calibration process is involved)(step S306); and (4) applying a suitable half-toning strategy, with reference to the color rendition dictionary, to convert the CMYK space device independent description to a device specific description (e.g., bits to be received by a raster output scanner or similar device for outputting the image) (step S308). See Henry R. Kang, "Color Technology for Electronic Imaging Devices" ISBN 0-8194-2108-1, SPIE Optical Engineering Press, 1997.
In processing the image, the critical step that accounts for variations in marking device output is TRC linearization. Tone Reproduction Curves are stored plots of an input parameter value versus an output parameter value for a particular color. A TRC is a monitonically increasing marking device function in input-output contone space or input-output density space or input-output byte space, or combinations thereof In other words, a TRC indicates the value of the output parameter for a specific device that must be used to reproduce the input parameter (if the input and output parameters are exactly equal, then the inputs and outputs are expressed in the same coordinate space). Inaccuracies in the TRC linearization step because of infrequent processing lead to inaccuracies in gray balancing and the 3D look-up table. The inaccuracies in the 3D look-up table can also result because the most current marking device information is not considered.
It would be desirable to correct the TRC linearization on a real time basis before that linearization is used to create output. As a result, the output would be accurate and would not vary. It would be especially desireable to provide a real time correction of page to page deviations in output. Updating the TRC linearization would require reprogramming already-programmed marking device drivers, which is difficult. Thus, it would be desireable to correct the output outside the marking engine as opposed to inside the marking engine.